The present invention relates in general to a radio receiver with improved reproduction quality in the presence of adjacent-channel interferers and/or noise sources, and more specifically, to separately-controlled adaptive filtering of the intermediate frequency signal and of the baseband (audio) signal.
In superheterodyne receivers, an RF signal from an antenna is processed through a series of mixing (frequency translation) and filtering operations. Due to all the various types of reception conditions that are possible (e.g., noise and interference), trade-offs have to be made in designing a filtering scheme so as to provide satisfactory reception under most conditions.
Commercial AM and FM broadcast bands include a plurality of evenly spaced channels. A particular broadcast station is allocated a unique channel to conduct broadcasting within an assigned frequency range.
The power spectrum of a broadcast transmission consists of the strength of RF signals irradiating a receiving antenna over a range of frequencies. Even though most energy in a transmission can be limited to its assigned channel, some energy is often radiated at frequencies outside their assigned channels. Adjacent and/or alternate channel interferers are often present. Radio receivers must be able to perform adequately in situations where a strong signal on an adjacent or alternate channel creates signal components in the desired channel which interfere with reception of the desired signal.
Prior art radio receivers are known which detect the presence of interference in various ways and which attempt to reduce effects of the interference by narrowing the receiver bandwidth (or by shifting the receiver passband for the desired channel). Separate filters or retuning of the receiver are typical methods for detecting presence of an interfering channel.
Other prior art receivers are known which provide dynamic noise reduction by adaptively filtering the baseband (i.e., demodulated) signal according to the frequency content of the baseband signal. The bandwidth at baseband is widened or narrowed as necessary to closely match the frequency content of the received signal. High frequencies at which low levels of energy are measured are assumed to contain only noise and are, therefore, filtered out.
Even though all channels in a broadcast service may be assigned equal bandwidth, not all transmitters utilize the available bandwidth to the same extent. For example, the standard NRSC-1 AM Preemphasis/Deemphasis and Broadcast Audio Transmission Bandwidth Specifications (ANSI-EIA-549-1988), which was published by the National Radio Systems Committee, provides for full utilization of AM channel bandwidth and for a specified preemphasis characteristic by the transmitter. A specially designed receiver (which may be certified as an AMAX receiver) can provide specified filtering and deemphasis characteristics of its own in order to take advantage of the potential reception improvements. However, such a receiver should not be limited to only those fixed characteristics since not all AM stations use all aspects of the standard in their broadcasts.
In view of all the different filtering criteria to reduce noise and interference and all the different broadcast characteristics that may be present in a desired signal, a filtering system is needed which is compatible with various types of broadcasts but which is effective in removing noise and interference in all cases.